Your email exceeds the maximum message size allowed by the recipient server. This includes all content: headers, body text, HTML, and attachments after Base64 encoding. Reduce the message size or use file sharing links.
What Does Error 5.2.3 Mean?
Enhanced status code 5.2.3 is functionally similar to SMTP code 552 and indicates your message exceeds the size limit set by the recipient mail server or organization. The limit varies by provider - Gmail accepts 25MB, some corporate servers may limit to 10MB or less.
Remember that Base64 encoding increases attachment sizes by approximately 33%. A 20MB file becomes about 27MB after encoding. Additionally, HTML emails with many embedded images can be surprisingly large. Always check total message size including encoding overhead.
Common Causes
- Attachments too large for the recipient server limit
- HTML email with many embedded images
- Base64 encoding overhead pushing message over the limit
- Organization has set restrictive size limits
How to Fix Error 5.2.3
- Reduce attachment sizes - compress files or lower image resolution
- Use cloud file sharing links instead of attachments
- Strip unnecessary embedded images from HTML templates
- Check the recipient server SIZE parameter in the EHLO response
Frequently Asked Questions
Bounce code 5.2.3 means your email exceeded the per-mailbox message size limit set by the recipient's administrator. This is distinct from the system-wide size limit (5.3.4) -- it applies to a specific mailbox that has a stricter size restriction than the server's general limit. The email cannot be delivered in its current size and must be reduced before resending.
Reduce your email's total size by compressing attachments, using cloud storage links (Google Drive, OneDrive, Dropbox) instead of direct attachments, or splitting the message into multiple smaller emails. Remember that MIME encoding increases actual message size by approximately 37% over the raw file size, so a 25 MB attachment becomes roughly 34 MB in transit. Contact the recipient to confirm their specific size limit and adjust accordingly.
Error 5.2.3 indicates the message exceeded a per-mailbox administrative size limit specific to the recipient's account, while 5.3.4 indicates the message exceeded the entire mail system's fixed maximum size. In practice, 5.2.3 means the specific recipient has a lower size limit than the server default, while 5.3.4 means the message is too large for the server overall. Both require reducing message size, but 5.2.3 may only affect certain recipients on the same server.